1. Field of the Invention
The present invention relates to a device, a program and a method to detect objects in the ocean for example, using a phase-conjugate pseudo sound sweeping system.
2. Related Art
Attempts to detect objects have been made in various technical fields. For instance, a sonar device is generally used for detecting objects (submarines, fishes, etc.) in the ocean.
Although a sonar device is also used for detecting objects (sunken ships, submarine cables, mines, etc.) buried in the sea bottom, a use is limited because the acoustic characteristics of the seawater and those of submarine sediments are different. That is, the seawater is liquid, but sediments on the sea bottom are in a solid state (but not a complete solid). An underwater sound wave (hereinafter, referred to as sound wave) has such a characteristic that when it is made incident on the boundary between the water and a submarine sediment, that is, made incident on the sea bottom, a part of the sound wave enters into the submarine sediment, although a part of the sound wave is reflected (reflected wave) at the sea bottom at the boundary. In this way, the sound wave is divided into three waves, including a transverse wave component (transverse wave) and a lateral wave component (lateral wave) besides a longitudinal wave component (longitudinal wave) which is the same as the water. If the incident angle of the sound wave is larger than the critical angle, the sound wave is totally reflected at the sea bottom without entering into the submarine sediment. A lateral wave is generated when the incident angle is larger than the critical angle.
As these three waves have different propagation speeds and propagating directions respectively, waves which strike an object buried in the submarine sediment (buried object) and reflect are also different, respectively. Therefore, in a wave receiver in the water, a reflected wave from the surface of the sea bottom and three wave components reflected at the sediment are complicatedly interfered and received. Thereby, detecting a buried object in the sea bottom involves difficulties (see “Scattering by objects buried in underwater sediments: Theory and experiment” by R. Lim, J. L. Lopes, R/H/Hackman, and D. G. Todoroff, J. acoust. Soc. Am. 93, 1762-1783 (1993) (Non-Patent Document 1), “A Model for high-frequency acoustic backscatter from gas bubbles in sandy sediments at shallow grazing angles”, by F. A. Boyle and N. P. Chotiros, J. Acoust. Soc. Am., 98, 531-541 (Non-Patent Document 2), and “The scattering of a low-angle pulse beam from seafloor volume heterogeneities” by S. A. Swift and R. A. Stephen, J. Acoust. Soc. Am. 96, 991-1001 (Non-Patent Document 3)).
The above-described difficulties involved in detection can be solved by performing detection on conditions that a transverse component and a lateral component are not generated. In other words, when a sound wave is made incident on the sea bottom at a vertical or almost vertical angle, neither a transverse component nor a lateral component are generated. Therefore, conventional methods of searching a buried object using sub bottom profiler for example usually scan the search area while emitting a sound wave immediately downward from a ship or a submarine.
However, in the above-described searching method, an enormous time is required for the search to scan the search area without limitation. Further, in the case that the buried object is explosive such as a mine, emitting a sound from right above will harm the survey vessel, so that the buried object cannot be detected.